1.Field of the Invention
This invention relates to rubber compositions suitable for use in pneumatic radial tires.
2. Prior Art
Resource and energy saving has been a great concern during the recent years, and to meet with this trend, automobile tire manufacturers have directed their research and development efforts to reducing hysteresis loss of tire materials to a minimum possible extent and at the same time to providing light-weight tire products by reducing the amount of rubber materials used for the various portions of the tire. From the functional point of view of an automobile tire, greater efforts have been paid in minimizing hysteresis loss of inter alia the tread rubber compositions, but this has nearly reached its critical point and hysteresis improvements are now being sought in other portions of the tire such as sidewalls so as to reduce its rolling resistance.
The sidewall portion of a tire has an important roll to protect the tire carcass against external damage, and the rubber material for that tire portion is required to possess such mechanical properties as resistance to flex and crack growth and resistance to abrasion on contact for example with curbstones and other obstructions on the road.
Polybutadiene rubber (BR) less abundant in 1,2-vinyl bonds is known to exhibit high resistance to flex and crack growth, whilst natural rubber (NR) is strong against external wear.
Generally, the sidewall of a passenger car tire has an elastomer content of NR and BR in a ratio of preferably 3/7 - 7/3 depending upon the particular tire performance required.
Most frequent source of damage to the sidewalls of passenger cars is found in abrasion with the curbstone of the road, but such damage is relatively minor because the speed of the car is normally slow when rubbing against the curbstone as compared to heavy load trucks. Therefore, it is more to flex and crack growth than to external damage that attention should be directed from the safety point of view.
The sidewalls of the passenger car tire have been constructed with sometimes HAF but more widely with carbonblack of a relatively low reinforcement such as GPF and FEF as rubber materials containing such carbonblack present relatively small hysteresis loss. However, it has now been found imperative to minimize the use of carbonblack for the sidewall of the tire so as to reduce the rolling resistance in an effort to achieve the ultimate objective of resource and energy saving. This in turn leaves the problem that reduction in carbonblack will necessarily lead to vulnerability of the sidewall to external damage.